Great effort has been expended in the development of chemical sensors which can measure the presence and/or concentration of chemical species in blood or other biological fluids. These sensors can be macroelectrodes (nonmicrofabricated) of the everyday bench top variety for measuring the pH of samples, and they may sometimes take the form of microelectrodes suitable for implantation within the body of a subject. Such devices are presently made individually or in certain cases by a combination of hand assembly and manufacturing methods which may include the thin-film and photoresist techniques currently used to manufacture integrated circuits (See, for example, Pace, S., Sensors and Actuators 1982, 1, 475; Zemel, J.N., U.S. Pat. No. 4,302,530 in which is disclosed a method for fabricating a "substance-sensitive" photodefinable layer over semiconductor devices, especially ion-selective field effect transistors (ISFET)). In spite of this considerable and continuous effort, sensors based upon this ISFET technology have not become common articles of commerce. The fact is that wholly microfabricated biosensors, that is, sensors which are uniformly mass produced solely by thin-film techniques and the micromanufacturing methods, useful in the clinical setting and adaptable to the detection and measurement of a whole host of chemical and biological species, have not been manufactured successfully.
It is apparent that the degree of complexity involved with the mass production of commercially viable biosensors is much more formidable than even those persons of ordinary skill in the art once perceived. Of major concern is the compatibility of inherently harsh physical and chemical processes, associated with existing semiconductor manufacturing methods, with sensitive organic compounds and labile biologically active molecules which comprise part of a functioning biological sensor. An article by Eleccion (Eleccion, M. Electronics 1986, June 2, 26-30) describes the current state of affairs with regard to microsensors and makes brief references to active areas of research including the detection of specific ions, gases, and biological materials. Progress in the area of field effect transistors (FETs) is noted and problems and limitations with present manufacturing methods are discussed.
Numerous other review articles describe a variety of electrochemical devices including ion-selective electrodes (ISEs) and ISFETs which incorporate enzymes or immunoactive species (See, for example, Pinkerton, T.C. and Lawson, B.L. Clin. Chem. 1982, 28(9), 1946-1955; Lowe, C.R. Trends in Biotech. 1984, 2(3), 59-65; Koryta, J. Electrochim. Acta 1986, 31(5), 515-520; DeYoung, H.G. High Tech. 1983, Nov., 41-50; Davis, G. Biosensors 1986, 2, 101-124 and references cited therein). Also, the general principles of operation of enzyme-based sensors have been reviewed (See, Carr, P.W. and Bowers, L.D. Immobilized Enzymes in Analytical and Clinical Chemistry, Wiley-Interscience (1980). Various mathematical models of operation have been examined, including the external mass-transfer model by Racine, P. and Mindt, W. Experientia Suppl. 1971, 18, 525. Significant problems and limitations in the fabrication of these devices remain unconquered, however, especially with regard to the fabrication of sensors intended for the analysis of nonionic species. The mass production of biosensors based upon ionselective electrodes (ISEs) would be particularly useful as these sensors can be adapted easily for the analysis both of ionic as well as uncharged analyte species.
It is also important to note that in current clinical settings medical practitioners commonly request that analyses of one or more components of a complex biological fluid such as whole blood. Currently, such analyses require a certain amount of processing of the whole blood, such as filtration and centrifugation, to avoid contamination of the instruments or to simplify subsequent measurements. Frequently, blood samples are sent to a remote central facility where the analyses are performed. Patients are thus deprived of valuable information which, in most cases, is not available for hours, sometimes days. Clearly, substantial advantages can be envisaged if analyses on undiluted samples can be carried out and if instruments or sensors can be produced which can perform real-time measurements.